Kant thought that human beings occupy a special place in creation. He also believed that morality can be summed up in one ultimate principle, from which all duties and obligations are derived. Kant defined an imperative as any proposition that declares a certain kind of action (or inaction) to be necessary. A hypothetical imperative would compel action under a particular circumstance: If I wish to satisfy my thirst, then I must drink this lemonade. A categorical imperative would denote an absolute, unconditional requirement that exerts its authority in all circumstances, and is both required and justified as an end in itself. It is best known in its first formulation:

Act only according to that maxim by which you can at the same time will that it would become a universal law.

He expressed extreme dissatisfaction with the moral philosophy of his day because he believed it could never surpass the level of hypothetical imperatives. For example, a consequentialist standard may indicate that murder is wrong because it does not maximize good for the greatest number; but this would be irrelevant to someone who is not interested in maximizing the good. Consequently, Kant argued, hypothetical moral systems cannot persuade moral action or be regarded as bases for moral judgments against others, because the imperatives they are based on rely too heavily on subjective considerations.

A deontological moral system based on the demands of the categorical imperative was presented as an alternative.

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The nature of a moral proposition ("It is wrong to commit murder") must necessarily mean that a particular act or kind of act ought not be carried out under any circumstance ("One ought not commit murder"). This is the central point of his meta-ethical theory that establishes Kant as an extreme moral objectivist. A categorical imperative is the one and only basis for all moral statements, because a hypothetical imperative would depend on the subjective desires of the rational actors, rendering it powerless to compel moral action.

In contrast to David Hume, Kant viewed the human individual as a rationally autonomous self-conscious being with full freedom of action and self-determination. For a will to be considered "free", we must understand it as capable of effecting causal power without being caused to do so. But the idea of lawless free will, that is, a will acting without any causal structure, is incomprehensible. Therefore, a free will must be acting under laws that it gives to itself.

Although Kant conceded that there could be no conceivable example of free will, because any example would only show us a will as it appears to us — as a subject of natural laws — he nevertheless argued against determinism. He proposed that determinism is logically inconsistent: The determinist claims that because A caused B, and B caused C, that A is the true cause of C. Applied to a case of the human will, a determinist would be arguing that the will does not have causal power because something else had caused the will to act as it did. But that argument merely assumes what it set out to prove; that the human will is not part of the causal chain.

Secondly, Kant remarks that free will is inherently unknowable. Since even a free person could not possibly have knowledge of his own freedom, we cannot use our failure to find a proof for freedom as evidence for a lack of it. The observable world could never contain an example of freedom because it would never show us a will as it appears to itself, but only a will that is subject to natural laws imposed on it. But we do appear to ourselves as free. Therefore he argued for the idea of transcendental freedom — that is, freedom as a presupposition of the question "what ought I to do?" This is what gives us sufficient basis for ascribing moral responsibility: the rational and self-actualizing power of a person, which he calls moral autonomy: "the property the will has of being a law unto itself".

Since considerations of the physical details of actions are necessarily bound up with a person's subjective preferences, and could have been brought about without the action of a rational will, Kant concluded that the expected consequences of an act are themselves morally neutral, and therefore irrelevant to moral deliberation. The only objective basis for moral value would be the rationality of the Good Will, expressed in recognition of moral duty.

Duty is the necessity to act out of reverence for the law set by the categorical imperative. Because the consequences of an act are not the source of its moral worth, the source must be the maxim under which the act is performed, irrespective of all aspects or faculties of desire. Thus, an act can have moral content if, and only if, it is carried out solely with regard to a sense of moral duty; it is not enough that the act be consistent with duty, but carried out to achieve some particular interest.

From this step, Kant concludes that a moral proposition that is true must be one that is not tied to any particular conditions, including the identity of the person doing the moral deliberation. One could not morally command others by saying "It is wrong for you to murder, but it is not wrong for me to murder" because that would be a hypothetical imperative: Effectively saying "If I am person A, murder is right; If I am person B, murder is wrong". Therefore, a moral commandment must have universality, which is to say that it must be disconnected from the particular physical details surrounding the proposition, and could be applied to any rational being. This leads to the first formulation of the categorical imperative:

"Act only according to that maxim by which you can at the same time will that it would become a universal law."

First, we have a perfect duty not to act by maxims that result in logical contradictions when we attempt to universalize them. The moral proposition A: "It is permissible to steal" would result in a contradiction in conceivability because the notion of stealing presupposed the existence of property. But were A universalized, then there could be no property, and the proposition has logically annihilated itself. Hence we have a perfect duty never to steal. Similarly, if the moral proposition B: "It is permissible to lie" were true, there must be language, but the universalization of lying would destroy the meaning of language. Therefore proposition B results in a logical contradiction, and Kant (rather famously) declared that lying is impermissible in any and all conceivable circumstances.

Second, we have imperfect duty, which is the duty to act only by maxims that we would desire to be universalized. Since it depends somewhat on the subjective preferences of mankind, this duty is not as strong as a perfect duty, but it is still morally binding. The moral proposition C: "One should never lend aid to another person unless there is something in it for oneself" could only be morally true if no one ever wanted help from another person, because that is the only case in which we could will it to be true. Since we can determine (by empirical observation) that this is not the case, C results in a "contradiction of the will", and Kant claims we have an imperfect duty to help others in their times of need, when possible.

All rational action must set before itself not only a principle, but also an end. Most ends are of a subjective kind, because they need only be pursued if they are in line with some particular hypothetical imperative that a person may choose to adopt. For an end to be objective, it would be categorically necessary that we pursue it.

The free will is the source of all rational action. But to treat it as a subjective end is to deny the possibility of freedom in general. Because the autonomous will is the one and only source of moral action, it would contradict the first formulation to claim that a person is merely a means to some other end, rather than always an end in his or her self.

On this basis, Kant derives second formulation of the categorical imperative from the first.

"Act in such a way that you always treat humanity, whether in your own person or in the person of any other, never simply as a means, but always at the same time as an end."

By combining this formulation with the first, we learn that a person has perfect duty not to use itself or others merely as a means to some other end. As a slaveowner would be effectively asserting a moral right to own a person as a slave, he or she would be asserting a property right in another person. But this would violate the categorical imperative because it denies the basis for there to be free rational action at all; it denies the status of a person as an end in himself. One cannot, on Kant's account, ever suppose a right to treat another person as a mere means to an end.

The second formulation also leads to the imperfect duty to further the ends of ourselves and others. If any person desires perfection in himself or others, it would be his moral duty to seek that end for all persons equally, so long as that end does not contradict perfect duty.

Because a truly autonomous will would not be subject to any particular interest, it would only be subject to those laws which it makes for itself. But it must also regard those laws as if they would be binding to others, or they would not be universalizable, and hence they would not be laws of conduct at all. Thus Kant presents the notion of the hypothetical Kingdom of Ends of which he suggests all persons should consider themselves both members and heads.

"So act as though you were through your maxims a law-making member of a kingdom of ends."

We ought to act only by maxims which would harmonize with a possible kingdom of ends. We have perfect duty not to act by maxims that create incoherent or impossible states of natural affairs when we attempt to universalize them, and we have imperfect duty not to act by maxims that lead to unstable or greatly undesirable states of affairs.

An objection based on the coherence of Kant's position has been stated by Onora O'Neill (1993):

"This [most central] objection is that Kant's basic framework is incoherent. His account of human knowledge leads to a conception of human beings as parts of nature, whose desires, inclinations and actions are susceptible of ordinary causal explanation. Yet his account of human freedom demands that we view human agents as capable of self-determination, and specifically of determination in accordance with the principles of duty. Kant is apparently driven to a dual view of man: we are both phenomenal (natural, causally determined) beings and noumenal (non-natural, self-determining) beings. Many of Kant's critics have held that this dual-aspect view of human beings is ultimately incoherent."

This objection is based on the view that human free will is incompatible with a deterministic world of cause and effect. The question of free will is contested in philosophical debates and literature even today.

Although Kant was intensely critical of the use of examples as moral yardsticks, because they tend to rely on our moral intuitions (feelings) rather than our rational powers, this section will explore some interpretations of the categorical imperative for illustrative purposes.

Kant asserted that lying, or deception of any kind, would be forbidden under any interpretation and in any circumstance. In Groundwork, Kant gives the example of a person who seeks to borrow money without intending to pay it back. The maxim of this action, says Kant, results in a contradiction in conceivability (and thus contradicts perfect duty) because it would logically contradict the reliability of language. The right to deceive could also not be claimed because it would deny the status of the person deceived as an end in himself. And the theft would be incompatible with a possible kingdom of ends. Therefore, Kant denied the right
to lie or deceive for any reason, regardless of context or anticipated consequences.

Kant argued that any action taken against another person to which he or she could not possibly consent is a violation of perfect duty interpreted through the second formulation. If a thief were to steal a book from an unknowing victim, it may have been that the victim would have agreed, had the thief simply asked. However, no person can consent to theft, because the presence of consent would mean that the transfer was not a theft. Since the victim could not have consented to the action, it could not be instituted as a universal law of nature, and theft contradicts perfect duty.

Kant applied his categorical imperative to the issue of suicide in Groundwork of the Metaphysic of Morals, writing that:

If a man is reduced to despair by a series of misfortunes and feels wearied of life, but is still so far in possession of his reason that he can ask himself whether it would not be contrary to his duty to himself to take his own life, he should ask himself a question. He should inquire whether the maxim of his action could become a universal law of nature. His maxim is: From self-love I adopt it as a principle to shorten my life when its longer duration is likely to bring more evil than satisfaction. It is asked then simply whether this principle founded on self-love can become a universal law of nature. Now we see at once that a system of nature of which it should be a law to destroy life by means of the very feeling whose special nature it is to impel to the improvement of life would contradict itself, and therefore could not exist as a system of nature; hence the maxim cannot possibly exist as a universal law of nature, and consequently would be wholly inconsistent with the supreme principle of all duty.

A Kantian mode of thinking states only rational and autonomous beings are held to have intrinsic worth, and objects or creatures that (according to Kant) are not autonomous are held to have no intrinsic moral value.

Although actions with respect to non-rational agents do not have intrinsic moral content, Kant derived a prohibition against cruelty to animals as a violation of a duty in relation to oneself. According to Kant, man has the duty to strengthen the feeling of compassion, since this feeling promotes morality in relation to other human beings. But, cruelty to animals deadens the feeling of compassion in man. Therefore, man is obliged not to treat animals brutally (Kant, Metaphysics of Morals, § 17).

One of the first major challenges to Kant's reasoning came from the Swiss philosopher Benjamin Constant, who asserted that since truth telling must be universal, according to Kant's theories, one must (if asked) tell a known murderer the location of his prey. This challenge occurred while Kant was still alive, and his response was the essay On a Supposed Right to Tell Lies from Benevolent Motives. In this reply, Kant agreed with Constant, and argued that it is indeed one's moral duty to be truthful to a murderer.

Kant argued that telling the truth to the murderer is required because moral actions do not derive their worth from the expected consequences. He claimed that because lying to the murderer would treat him as a mere means to another end, the lie denies the rationality of another person, and therefore denies the possibility of there being free rational action at all. This lie results in a contradiction in conceivability and ergo the lie is in conflict with duty.

Furthermore, Kant questioned our ability to know that the expected future outcomes of our actions would actually occur. For example, suppose Jim said that the victim was in the park, when he thought the target was in the library. However, unbeknownst to Jim, the victim actually left the library and went to the park. The lie would actually lead the murderer to the victim, which would make Jim responsible for the murder. Another example post-Kantians bring up is that one would not be morally responsible for the action anyway; the murderer would be. If one told the truth, it might turn out the murderer decides not to murder after all[citation needed].

Because Kant's theory only draws distinctions between right acts and wrong acts, some question how the categorical imperative can explain the prioritization of moral duties when they come into conflict[citation needed]. For example, if one must steal in order to keep a promise, how should he or she act? Some Kantians have argued that promising to steal is an immoral act itself, because the declaration of intent to act immorally is inconsistent with free will. The fact that one has acted immorally in the past would not release him or her from the duty to act morally in the future; therefore, when confronted with the issue of choosing between stealing or promise-breaking, breaking the promise would not be an immoral act, but stealing would.

Another objection to Kant came from the Englishman, Sir David Ross, argued that a world where everyone could be depended upon to always break their promises would be just as effective and reliable as a world where everyone kept their promises, and one could thus will that promise-breaking become universalisable[citation needed]. However, this argument may fail to take into account Kant's argument that lying would violate the second formulation of the categorical imperative, which forbids treating another person as a means to another end.

Ayn Rand's critique amounts to this reductio ad absurdum: The deduction that the entire human race has a duty to die is entirely consistent with the Categorical Imperative provided that the deducer agrees that he himself, or she herself, has a duty to die too - regardless of any, some or all's inclination, rooted in self-love, to stay alive[citation needed]. At the root of her critique is her conclusion that Kantian appeals to the good implicitly appealed to Aristotle's eudaimonia while explicitly denying its relevance to morals. An example: a duty to promote universal war is exactly the same as a duty to promote universal peace once eudaimonia is removed.

Lewis White Beck argued that within Kant's theory, it is unclear what is a moral maxim and what is a prudential maxim[citation needed]. For example the maxim that the purchaser of every new book should write their name on the flyleaf: There is nothing in the categorical imperative to discern that this is not a moral imperative for it is easily something which one would wish to be universally applied, and this universal application would lead to no irrational contradictions. Of course this imperative is actually hypothetical, but the condition is merely omitted. One could say that you should always inscribe your name inside a new book, if you want it to be returned. The categorical imperative on its own cannot differentiate between a prudential maxim and one that is truly moral--this requires a longer and more complex method of reasoning.